A device for the automatic control of the pulling force of a cable for a snow-trail grooming device is known from the European Patent Application EP 1 118580 A1, where a pressure in the hydrostatic drive of the snow-trail grooming vehicle and an angle of the cable relative to the vehicle are detected. Using these detected magnitudes, a desired value for the pulling force of the cable is determined. The thus obtained desired value is compared with a detected actual value of the pulling force of the cable and the pulling force is controlled to the desired value in a closed control system. The pressure in the hydrostatic drive is separately detected according to the pressure on the forward side of the drive and according to the pressure on the rearward side of the drive. When the winch cable pulls, for example, forwardly in traveling direction and at the same time a high pressure bears on the forward side of the drive, then the desired value for the winch pulling force is chosen correspondingly higher in order to relieve the drive. Whereas when a high pressure bears on the rearward side of the drive during a cable pull in a forwardly traveling direction, then this means that the cable winch works against the drive. The winch pulling force should be reduced in this case. If, when traveling uphill, a drive chain or track suddenly loses traction, then the pressure on the forward side of the drive is automatically reduced. This results in a reduction of the pulling force of the cable even though, based on the occurring slip, a higher pulling force of the cable would be needed.
A quickly reacting and reliable control of a cable winch is to be achieved with the invention.
According to the invention a method for controlling a cable winch of a snow-trail grooming vehicle is provided with the following steps:                detecting a chain or track speed or a magnitude proportional to the chain or track speed,        detecting a cable speed or a magnitude proportional to the cable speed,        comparing the detected chain or track speed and the detected cable speed or the magnitudes proportional thereto, and        determining a desired value for the cable speed or the magnitude proportional thereto taking into consideration the comparison result for controlling the cable winch.        
By utilizing a chain or track speed and a cable speed for controlling the cable winch, a quick and reliable control can be realized since the control of the cable winch occurs principally independently from the pulling force of the cable. This is particularly advantageous in the case of long cables, which are used for snow-trail grooming vehicles, since with long cables, due to the at all times existing cable elasticity, the tension in the cable is subjected to great fluctuations. In the case of a loss of traction of a driving chain or track, the measured chain speed is increased so that also the cable speed is at least on short notice increased. Thus, it can be assured that when the driving chains or tracks lose traction the pulling force of the cable winch is increased on short notice in order to guarantee a steady forward movement of the snow-trail grooming vehicle.
A further development of the invention takes into consideration, when determining the desired value, a detected actual pulling force in the cable and a specifiable highest value for the pulling force.
A pulling force limitation of the cable can in this manner be taken into consideration and it can be assured that a maximum permissible pulling force is not exceeded. If necessary, the permissible highest value can be specified, for example, according to environmental conditions, age of cable, type of cable and also type of vehicle.
A further development of the invention detects a chain or track speed and a cable winch speed.
A speed of the driving chains or tracks and a speed of a cable drum or a guide roller with known diameter can be detected in a simple and reliable manner and is proportional with respect to the chain or cable speed.
A further development of the invention provides that the cable speed or its magnitude proportional thereto is detected in accordance with its amount and its direction relative to a traveling direction or a longitudinal axis of the snow-trail grooming vehicle.
A consideration of the cable pull angle is sensible, for example, when a cable winch pulls at an angle with respect to the traveling direction or with respect to the longitudinal axis of the snow-trail grooming vehicle. This is the case when, for example, in the case of flat intermediate pieces, the cable is to be kept taut.
A further development of the invention provides that, when determining the desired value, an angle of slope between the traveling direction or the longitudinal axis of the vehicle and the horizontal and a direction of the cable speed relative to the traveling direction are taken into consideration.
A cable winch control in dependency of the traveling direction on a slope can occur in this manner. For example, when traveling downhill and when the direction of pull is against the traveling direction, the desired value for the cable speed is chosen lower or equal to the chain or track speed in order to achieve a braking action. When traveling uphill and with a pulling direction of the cable in traveling direction, the desired value for the cable speed is chosen greater or equal to the chain or track speed in order to maintain a pulling force in the cable and to support the drive. The cable is basically supposed to be kept taut and under tension in all traveling situations.
A slip of the chains or tracks of the snow-trail grooming vehicle is detected and the detected slip is taken into consideration when determining the desired value according to a further development of the invention.
For example, when noticing an excessive slip at the driving chains or tracks it is possible to increase a desired value for the cable speed so that the pulling force of the cable is increased and the snow-trail grooming vehicle is pulled over the critical area with the help of the cable winch.
The basic problem of the invention is also solved by providing a method for controlling a cable winch of a snow-trail grooming vehicle comprising the following steps:                detecting a traveling speed above ground of the snow-trail grooming vehicle according to direction and amount,        detecting a cable speed according to direction and amount or a magnitude proportional to the cable speed,        comparing the detected traveling speed above ground and the detected cable speed or the magnitude proportional thereto, and        determining a desired value for the cable speed or the magnitude proportional thereto for controlling the cable winch taking into consideration the comparison result.        
A traveling speed above ground and the cable speed can be directly compared since the cable of a cable winch is anchored at terrain-fixed points. A traveling speed above ground can be determined with good precision, for example, by means of a satellite navigational system alone or by coupling of several navigational systems.
The basic problem of the invention is also solved by providing a snow-trail grooming vehicle having a cable winch for carrying out the inventive method, which has means for detecting a chain or track speed, a magnitude proportional thereto and/or a traveling speed above ground, means for detecting a cable speed or a magnitude proportional thereto, means for comparing the detected cable speed and the detected chain or track speed or the magnitudes proportional thereto and/or the traveling speed above ground, and means for determining a desired value for the cable speed taking into consideration the comparison result.